Skin is typically characterized as consisting of three distinct layers, namely the stratum corneum, the epidermis and the dermis. The stratum corneum, the outermost layer, is made up of keratinized cells, surrounded by intercellular space filled with lipids. The stratum corneum provides a substantial physical barrier to penetration of most substances to the lower layers of the skin. In addition to preventing transport of substances to the other skin layers, however, this barrier also aids in prevention of water loss from the skin. Both functions are primarily attributable to the presence of the lipids in the stratum corneum.
There are two sources of the skin surface lipids making up this important barrier: sebaceous glands and the epidermis. The lipids are a diverse group of compounds, comprising triglycerides, diglycerides, ceramides, free fatty acids, wax esters, cholesterol and cholesterol esters, and squalene. The quantity and composition of the skin surface lipids differ from place to place on the body, and may to some extent be related to the number of sebaceous glands in a given area of the skin. The condition of the skin surface lipids may also be affected by an essential fatty acid deficiency. Additionally, the lipid barrier is easily diminished by exposure to harsh detergents or soaps and other environmental factors. It is apparent, then, that the quality of the skin lipid barrier can vary widely, depending on a number of different factors, and therefore, may not always be adequate to perform its protective function optimally.
As an attempt to compensate for what may be a less than adequate lipid barrier, cosmetic compositions frequently incorporate components which compensate for water loss. Examples of such materials are hygroscopic humectants, e.g., urea or propylene glycol; or emollients, e.g., oleyl alcohol or caprylic/capric triglycerides. Certain cosmetic components may be occlusive skin conditioners, which are used to provide an "artificial" barrier; such compounds are frequently hydrophobic materials which remain on the skin surface, and include various hydrogenated oils, waxes and butters. Although many of these products provide an effective means of stemming water loss from the skin, they do not generally constitute a natural-occurring component of the stratum corneum, potentially giving rise to an unnatural, greasy feel to the skin. In addition, various pharmaceutical or cosmetic active agents are also frequently used to treat the symptoms of dry skin-associated conditions; however, in many cases, particularly with pharmaceutical agents, the treatments themselves may cause undesirable side effects in the individual being treated, while ultimately resulting in no actual reconstitution or repair of the lipid barrier.
In recent years, considerable study has been devoted to efforts to determine the normal lipid composition of the skin. In order to repair or maintain the integrity of the barrier, it is important to know what the normal, fully functional state of the barrier should be. For example, Elias (J. Invest. Dermatol. 80(6)Supp.:44, 1983) discloses a naturally-occurring lipid composition, apparently from human abdomen epidermis, containing relatively large quantities of sphingolipids, free sterols, cholesterol sulfate, and free fatty acids. In particular, the fatty acids, particularly the fatty acid portions of the sphingolipid components, are said to be enriched in long-chain, fully saturated acyl groups. WO 94/00127 further discloses synthetic lipid compositions comprising as its main components cholesterol, ceramides, and fatty acids. Apparently based at least in part on the reported composition of the natural epidermal lipid barrier, these compositions are said to be useful in moisturizing and lipid barrier fortification and repair.
However, to date, there have been no studies conducted on lip lipid compositions. The lips provide a unique problem, in that, unlike other areas of the body, they are virtually always exposed to seasonal environmental hazards, such as wind, cold and sun, and are therefore susceptible to frequent chapping, cracking and dryness. In addition, many lip cosmetics, such as traditional lipsticks, may contribute to the dryness. Although there are lip products which are aimed at treating or preventing this problem, these are primarily of the "artificial barrier" type, and are not capable of repairing or reconstituting the natural lipid barrier. Thus, there continues to be a need for a treatment regimen and composition which specifically addresses the singular difficulties associated with maintaining and treating the lip area. The present invention now fills this need.